This invention relates to spray guns for liquids such as paint or the like and, more particularly, to a spray gun for hydraulically atomizing the liquid to be sprayed.
In a hydraulically atomizing spray system or "airless spray", the liquid is forced through a spray tip or nozzle opening having sharp edges at sufficiently high velocities to provide atomization of the liquid. The nozzle opening has an elliptically shaped cross-section with sharp vertices to produce an elliptical or oval shaped spray pattern. In airless spraying techniques, the hydraulic pressures are relatively high and typically range from about 1,500 psig to about 3,000 psig.
The advantages of airless spraying are discussed in U.S. Pat. No. 3,000,576 to Levey et al. In accordance with this prior patent, the non-uniform spray pattern having thickened portions or "pig-tails" which had theretofore characterized airless spray techniques was eliminated by the use of a restricted pre-orifice having a circular cross-section disposed upstream from the nozzle opening.
The improvements disclosed in U.S. Pat. No. 3,000,576 were founded upon the recognition of the fact that the occurrence of such nonuniform spray patterns was associated with the formation of a vena contracta upon acceleration of the liquid with a large pressure drop through the nozzle opening which prevents full atomization of the liquid at the vertices of the spray pattern. The formation of a harmful vena contracta was eliminated by accelerating the liquid with a large pressure drop through the restricted pre-orifice to provide a submerged jet moving through the nozzle opening by its own kinetic energy at substantially the same velocity. By controlling the liquid in this manner, a substantially uniform pressure and velocity was obtained across the area of the nozzle opening, so that the liquid was uniformly atomized and distributed across the spray pattern with a uniform reduction in density or feathering at the margins of the pattern.
For purposes of obtaining optimum atomization quality, the cross-sectional area of the pre-orifice opening should be substantially equal to the cross-sectional area of the nozzle opening. Satisfactory results are obtained as long as the area of the pre-orifice is not less than about one-fourth or more than about twice the area of the nozzle opening. A number of variously sized, pre-orifice units are required to accommodate the full commercial range of spray tip opening sizes in accordance with the teachings of the said U.S. Pat. No. 3,000,576. In order to maintain optimum atomization, the variously sized pre-orifice units must be changed and such may involve the disassembly of a significant portion of the spray gun and, at least, results in a considerable loss of production time.
Further, in some paint spraying applications, particularly in construction or maintenance work, speed of application is paramount and the quality of atomization may be sacrificed for the maximum fan width applied as quickly as possible. To that end, an operator may forego the use of a preorifice in order to obtain as wide a spray fan width as possible since it has been observed that the use of a pre-orifice will reduce the fan width and the volume delivered. Thus, a similar loss of production time may once again be encountered while the operator inserts a pre-orifice after the faster "rough" applications have been applied.
The necessity of employing a number of variously sized pre-orifice units and the loss of production time associated therewith have been eliminated by the adjustable pre-orifice assembly disclosed in applicants' co-pending U.S. Pat. application Ser. No. 443,480, filed Feb. 19, 1974. In accordance with the improvements set forth therein, a pre-orifice assembly or capsule provides a pre-orifice which is adjustable in cross-sectional area to accommodate the full commercial range of spray tips. The area of the pre-orifice opening is adjusted by means of a control member mounted on the external body portion of the spray gun. In addition to providing the optimum 1:1 ratio, the adjustable pre-orifice also provides area ratios substantially in excess of 2:1 for most of the spray tips commonly employed to enable an operator to achieve full spray pattern width potential in high speed rough work by simply adjusting the pre-orifice to its maximum size. Similarly, the spray fan width may be narrowed or decreased for specific trim or finishing applications by reducing the size of the pre-orifice.